This invention relates to forming a protective coating on a continuously moving metal strip. More particularly, this invention relates to forming a smooth plastic coating from electrostatically charged powder.
It is well known to continuously coat metal strip with solvent based paint. Painted metals can be fabricated by deep drawing, shaping, or roll forming into a variety of articles including building panels, lock seam culvert, appliance components, vehicular components and the like. The strip surfaces are cleaned and degreased and liquid paint is applied using a roll coater, gravure, dipping, spraying, electrocoating, and the like. The conventional manner of drying liquid paint is driving off the solvent using a long convention oven.
There are several disadvantages when using a solvent based paint. Convection heating is very inefficient because of poor heat transfer through the air between the oven heaters and the metal strip. This necessitates a very long oven and/or a very slow strip speed to dry the coating. Solvent fumes are an environmental concern requiring the oven to be enclosed to prevent release of the fumes into the work area. Certain types of fumes may have to be recycled to an incinerator for disposal. There also is environmental concern associated with maintaining the work area in and around the coating station. The wastes from cleaning the coating equipment and the work area may be hazardous and therefore must be disposed of properly. There also are several disadvantages with the coating itself. Only thin coatings generally can be produced and poor surface coverage is a problem when paint is applied to an embossed or coined metal surface. Since drying of the paint when using convection heating occurs from the outside toward the inside, blistering of the paint also may occur.
It is known to form pollution free thin plastic coatings on a metal surface using electrostatically charged powder that may be melted in a short period of time, i.e. less than one minute, using infrared heating. For example, U.S. Pat. No. 3,396,699 discloses continuously passing metal wire or strip through an enclosed chamber containing a suspended cloud of electrostatically charged plastic powder. An epoxy coating having a thickness of 38 microns is formed by passing powder coated wire through an infrared heated oven. U.S. Pat. No. 3,560,239 discloses plastic powder coating steel wire or strip by preheating using an induction coil, passing the steel through a fluidized powder coating chamber, melting the powder by passing the steel through another induction coil, and then water quenching the liquid coating. U.S. Pat. No. 4,244,985 discloses using a fluidized bed to coat metal tubing or wire with a thermosetting powder. The patent discloses thermosetting coatings having thicknesses in the range of 25-75 microns. Examples of induction coil heating times of 3-14 seconds are given.
It also is known to coat a metal surface with electrostatically charged powder using spray guns. U.S. Pat. No. 3,439,649 discloses electrostatic spray guns positioned inside an enclosed coating chamber for coating a preheated steel strip with plastic powder. A coating thickness of about 3-13 microns is disclosed when a perpendicularly directed spray gun is positioned about 15 cm above and below the strip surfaces. British patent 1,273,159 discloses positioning an inclined nozzle both above and below a moving metal strip for blowing a gas jet carrying plastic powder toward the strip. The powder is electrostatically charged using a wire grid positioned inside the coating chamber.
Nevertheless, there remains a need for a coating process for applying powder that can be melted to form a coating having a uniform thickness and whose surface is smooth and free of cosmetic imperfections. More particularly, there remains a need for forming a ductile thermosetting coating that is sufficiently cured to resist cracking and provide corrosion resistance when the coated metal strip is fabricated into an article. Furthermore, there remains a need to cure a thermosetting coating in a short period of time to minimize coating line length, the amount of space required, and to permit increased coating line speed.